Controlled percolation system and method for heap leach mining

ABSTRACT

A system for heap leach mining comprises supply and header pipes for conducting leaching solution alongside a bed or &#34;heap&#34; of crushed metal-laden ore deposited on an impervious pad. A series of spaced tubes are connected to the supply header pipes to extend transversely thereof, over the pad, to receive the leaching solution therefrom. A plurality of emitters are connected in spaced relationship along each of the tubes for emitting the leaching solution at a controlled and substantially uniform rate over the pad and into the ore bed. The tubes and emitters can be positioned either on an upper surface of the ore bed or buried beneath such surface, at a predetermined depth. The system and method of this invention provide numerous advantages over conventional surface spraying systems and methods, including substantial cost savings thereover.

This application is a Continuation of U.S. patent application Ser. No.07/116,465, filed on Nov. 3, 1987 by Richard G. Krauth for "ControlledPercolation System and Method for Heap Leach Mining."

TECHNICAL FIELD

This invention relates to hydrometallurgy and more particularly to theapplication of a leaching solution over a bed of crushed metal-laden oreto separate-out a pregnant solution of metals therefrom.

BACKGROUND OF THE INVENTION

Hydrometallurgy is the art of recovering metals from ores by firsteffecting solution of the metals in the form of a salt, separating thesolution from the impoverished solid, then decomposing the metallic saltin such a way as to cause precipitation of the metal in a state ofcomparative purity. The leaching solution or lixiviant for reclaiminggold and silver from a bed of low-grade ore generally constitutesaqueous solutions of sodium cyanide mixed with oxygen (air) to convertthe noble metal (M) to soluble NaM(CN)₂, from which M can be recoveredeither by precipitation with zinc dust or aluminum powder, carbonabsorption, or by electrowinning. Sulfuric acid or sulfuric acid-ferricsulfate is the principle practical copper mineral lixiviant.

The term "heap," as used in the art of hydrometallurgy, means a bed ofrun-of-mine or granular low-grade ore that has been suitably spread overa prepared surface or "pad" that will ensure solution recovery. The padis formed as an impervious base, such as by sheets of plastic film(e.g., polyethylene), asphalt and/or compacted clay. The ore is normallyheaped onto each impervious pad to a depth of ten to thirty feet, afterthe ore has been pre-crushed to sufficiently small size to enable theleaching solution to reach the metallic-mineral particles contained inthe ore. Gold and silver ores, for example, are run-of-mine or morecommonly crushed to 40-200 mesh size with recovery increasing with theincreased fineness of the crushed ore. The pad is usually sloped withthe pregnant solution recovered for subsequent processing by perforateddrain pipes or channels positioned on the impervious base, at the "toe"of the pad.

Lixiviation is the process effecting contact between the ore and theleaching solution. The process is normally accomplished by circulatingthe solution through the stationary ore mass, commonly known as"percolation." The rate of percolation will depend on a number offactors, such as the particle size of the ore, the depth of the pad, thestrength of the leaching solution (e.g., one to four pounds of sodiumcyanide per ton of water for recovering gold) and the quantity ofleaching solution distributed over the bed of crushed ore per unit area.

Conventional percolation systems for heap leach mining normally includesprayers mounted above the bed of crushed ore which function to spraythe leaching solution onto the ore in a non-uniform and uncontrolledmanner. The leaching solution is thus prone to rapid evaporation anddegradation by its exposure to ultra-violet rays, requiring constantmonitoring and replenishing of the leaching solution with added waterand/or sodium cyanide. Evaporation is further increased by exposure ofthe leaching solution to wind friction when it is sprayed through theair.

Surface spraying also induces ice build-up over the pad in freezingweather which in many instances requires complete operation shut-down ora risk of wash-out or blow-out, as will be appreciated by those skilledin the heap leach mining arts. As an alternative to shutting down theoperation, the leaching solution and/or the ore itself may be heated,but at substantial cost. The aforementioned wind factor may give rise toan unsafe working environment for the operating and maintenancepersonnel and also tends to contaminate the surrounding environment(air, ground water, soil) with the highly toxic leaching solution.

Further disadvantages of conventional surface spraying techniquesinclude the formation of surface ponding and run-off, giving rise toserious channeling problems and potential for blow-outs. Ice melt-downs,ponding and other adverse effects, resulting from uneven solutiondistribution common to surface spraying, will result in pad channeling.The fines washed-down by solution channels form hard pans thateventually divert and internally erode the ore bed and pad. Should theseconditions persist, pad blow-out will oftentimes result.

The sprayers used for surface spraying also require constant repair tounplug their spraying heads. Conventional systems of this type are alsounusable when the pad is situated over uneven terrain, due to theabove-mentioned channeling and pad blow-put problems.

SUMMARY OF THE INVENTION

A primary object of this invention is to provide an improved percolationsystem for heap leach mining which overcomes the above, brieflyenumerated disadvantages of conventional systems employing surfacespraying.

The controlled percolation system of this invention comprises supplymeans for conducting a leaching solution adjacent to an ore bed composedof crushed metal-laden ore, a plurality of spaced tube means connectedto the supply means for receiving the leaching solution therefrom, and aplurality of dripper means positioned in spaced relationship along eachof the tube means for directly emitting the leaching solution into theore bed at a controlled and substantially uniform rate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of this invention will become apparent fromthe following description and accompanying drawings wherein:

FIG. 1 is a perspective view partially illustrating an ore bed composedof crushed metal-laden ore deposited on an impervious pad and having acontrolled percolation system embodiment of this invention disposedthereon;

FIG. 2 is an enlarged sectional view in elevation, taken in thedirection of arrows II--II in FIG. 1, illustrating a header pipe and oneconnecting tube having a plurality of emitters for directly emitting aleaching solution onto the ore bed;

FIG. 3 is an enlarged perspective view illustrating connection of theheader pipe to a plurality of tubes;

FIG. 4 is an enlarged longitudinal sectional view of one of one type ofemitting unit suitable for use in the percolation system of thisinvention.

FIG. 5 is a cross-sectional view, similar to FIG. 2, but illustratingthe tube and emitters positioned beneath an upper surface of the orebed; and

FIG. 6 is similar to FIG. 1, but illustrates multiple layers of ore bedshaving the percolation system disposed thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

General Description

FIG. 1 illustrates a controlled percolation system 10 of this inventionmounted on the upper surface of an ore bed or "heap" 11 composed ofcrushed metal-laden ore, such as low-grade gold, silver or copperbearing ores. The ore bed is deposited on an pad 12, shown, for example,in the form of one or more plastic sheets 13 which can be placed over anasphalt or compacted clay base. As described above, heap leach mining istypically employed in conjunction with low grade ore that is normallypre-crushed to suitable size and then treated with a leaching solution(lixiviation), such as an aqueous solution of sodium cyanide, potassiumcyanide, acidothiourea, ammoniacal thiosulfate or malononitrile in thecase of gold and silver. It should be understood that the system andmethod of this invention are equally applicable to the reclamation ofother types of metals, such as copper wherein sulfuric acid or sulfuricacid-ferric sulfate is used as the lixiviant.

The illustrated embodiment of controlled percolation system 10 includessupply means in the form of a mainline pipe 14 and a plurality of headerpipes 15 for conducting the liquid leaching solution to separate seriesof tubes 16, suitably connected to the header pipes. Each tube has aplurality of spaced emitters 17 secured thereon for directly emittingthe leaching solution into ore bed 11 at a controlled and substantiallyuniform rate, in a manner described more fully hereinafter. The pregnantsolution is recovered by strategically positioned drain pipes 18 (oneshown in FIG. 1) positioned on pad 12. Thereafter, the metals, such asgold, silver or copper, are recovered from the pregnant solution in anywell-known conventional manner, such as by precipitation with zinc dustor aluminum powder, carbon absorption or by electrowinning.

DETAILED DESCRIPTION

Mainline pipe 14 is connected to a source of the liquid leachingsolution in a conventional manner to receive the solution at a suitablepressure (e.g., 10-100 psi). Emitters 17 may be constructed to benon-pressure compensating or pressure compensating and self-cleaning tomaintain a substantially uniform flow rate therefrom, independent ofupstream pressure fluctuations. The mainline and header pipes arepreferably constructed from a standard plastic material, such as asemi-rigid polyvinylchloride (PVC). A twelve to sixteen inch diametermainline pipe and six inch diameter header pipes have been foundacceptable for this purpose.

A series of tubes 16 are connected to one or both sides of a header pipe15 to extend transversely therefrom. In each series, the generallyparallel tubes are preferably spaced apart one from another at adistance selected from the approximate range of from 1.0 ft. to 12.0 ft.(FIG. 3). Tubes 16 are preferably composed of a flexible plastic tubing,such as low density polyethylene, that will exhibit sufficientflexibility to at least generally conform to an irregular upper surface19 of ore bed 11. Although the diameters of the tubes may vary, in oneworking embodiment of this invention, each tube had a standard outerdiameter of 0.620 in. and an inside diameter of 0.520 in. to facilitateassembly of the system from commercially available components.

As shown in FIG. 2, each tube is positioned on upper surface 19 of orebed 11 to emit the leaching solution directly into the ore, even thoughportions of the tubes and emitters 17 may be slightly out of contactwith the upper surface of the ore bed due to surface irregularities. Theterm "emit" (or "emitting") as used herein means a process fordispensing a leaching solution in such a manner that the solution flowsor drops down from the dispensing site without being projectedhorizontally to any substantial extent above ground level (such as insurface spraying) prior to contact with the bed of crushed ore. Theleaching solution will typically in a generally umbrella-shaped pattern20, with the adjacent patterns overlapping as shown. The solution willfurther penetrate and soak the volume of ore between the patterns underthe influence of capillary and related principles influencing fluidflow. Ore bed 11 is normally formed to a depth of from ten to thirtyfeet.

FIG. 4 illustrates one type of in-line emitter 17 that can be used incontrolled percolation system 10. As shown, the emitter comprises acylindrical outer housing 21 having a cylindrical inner flow controlmember 22 snap-fit and held therein by an interengaging flange andgroove arrangement 23. Tubes connect in sealing relationship to eitherside of the emitter 17 at cylindrical barbs 24 and 25.

This general type of emitter (further described in U.S. Pat. No.3,792,588) is commonly used for drip irrigation purposes. The emitterincludes an inlet 26 defined at the upstream end of member 22 tocommunicate the liquid leaching solution through a labyrinth-type flowpassage 27 in a conventional manner, i.e., the flow reverses directionlongitudinally, back and forth, throughout its travel through thepassages. An outlet 28 is defined at the upstream end of member 22 andpassage 27 to permit the solution to flow in a controlled manner fromthe passage to outlet 28 and thence through an outlet passage 29. Thesolution then seeps out at a predetermined flow rate between housing 21and member 22 for distribution over pad 11, as shown in FIG. 2. Atypical emitter of this type is manufactured by the OreMax Division ofWade Manufacturing Co. of Fresno, Calif. and can be pre-calibrated andconstructed to have a flow rate selected from the range of from 0.2 gphto 6.0 gph.

Other types of emitters can be substituted in lieu of emitter 17, solong as they provide the above-described controlled and substantiallyuniform rate of distribution of the leaching solution over the pad. Forexample, the well-known "In-Line," Micro-Flapper Emitter (Model MF) orthe "On-Line (Button) Emitter", all manufactured by Wade ManufacturingCo. of Fresno, Calif. may prove useful for this purpose. Emitters ofthese type are normally clog resistant (self-cleaning) and pressurecompensating to ensure continuous and uniform distribution of theleaching solution over the pad.

The composite average flow rate from the array of emitters 17 ispreferably at a rate selected from the approximate range of from 0.05 to1.50 gph/ft² of the surface area of ore bed 11. A more limited preferredrange is from 0.06 to 1.20 gph/ft². As suggested above, each emitter 17preferably has a flow rate selected from the approximate range of 0.2gph to 6.0 gph and still more preferably within the range of from 0.5 to5.0 gph.

As shown in FIG. 1, each series of tubes 16 and emitters 17 arepreferably positioned to form at least generally parallel linesseparated one from another at a distance selected from the approximaterange of from 1.0 ft. to 12.0 ft. The emitters are spaced apart one fromanother on each of the tubes at a linear distance preferably selectedfrom the approximate range of from 1.0 ft. to 12.0 ft. As shown in FIG.3, the majority of the emitters in one tube are preferably staggeredrelative to the emitters in the next adjacent, parallel line of emittersto further aid in the substantially uniform saturation of ore bed 11with the solution.

The rate at which the leaching solution is distributed over a unit areaof the pad will depend on a number of predetermined design and relatedfactors, such as the viscosity of the leaching solution, the size of theoutlet of each of the emitters and the spacing of the emitters relativeto each other. The required time of contact of the leaching solutionwith the ore bed will be largely determined by the degree ofpulverization of the ore, the strength of the leaching solution, thedegree of solubility of the mineral or the form in which it occurs, andthe effectiveness of the contact. Such contact time may be from fourhours to sixty days with many of the applications requiring from sevento forty-five days. In many instances the process can be accelerated byheating the leaching solution and/or by aeration wherein air is injectedand mixed with the solution prior to or during its distribution over thepad.

FIG. 5 illustrates positioning the major portions of tubes 16 andemitters 17 at least substantially beneath upper surface 19 of ore bed11, preferably at an approximate depth of from 4.0 in. to 72.0 in.Additional piles or heaps 30 of ore bed 11 can be used to cover headerpipes 15, as well as the tube portions and emitters disposed abovesurface 19. This modified system is particularly useful for thepercolation process when ambient temperatures fall below freezing.

FIG. 6 illustrates a further modification and use of the system whereinmultiple layers ("lifts") of underlying and previously processed orebeds 11' and 11" are covered with another ore bed 11, processed with theaddition of air injection. In particular, after first and second orebeds 11' and 11" have been sequentially processed in the above-describedmanner, newly deposited ore bed 11 can be processed in a like manner bysystem 10. Underlying systems 10' and/or 10" are left intact and can besupplied with injected pressurized air (in lieu of the leachingsolution) to enhance oxidation (arrows 31) of the leaching solutionemitted from system 10 (arrows 32). This added feature will not only aidin increasing the percolation efficiency of the leaching solution, butwill also aid in the further solubilization of metals from previouslyprocessed ore beds 11' and 11".

From the above description it can be appreciated that this inventionconstitutes a marked improvement over conventional percolation systemsfor heap leach mining which include sprayers mounted above an ore bed tospray a leaching solution onto the ore bed. The system and method ofthis invention function to emit and distribute a leaching solution in anuniform and controlled manner to render it far less susceptible toevaporation and degradation by its exposure to ultra-violet rays.Constant monitoring and replenishing of the leaching solution with addedwater and/or sodium cyanide is thus normally not required. Evaporationis further deterred since the bulk of the leaching solution is notexposed to wind friction.

Inducement of ice build-up over the pad in freezing weather is alsosubstantially eliminated (particularly with use of the FIG. 5 system)whereby the need for expensive supplemental heat input is eliminated andoperation shut-down or a risk of wash-out or blow-out do not poseproblems. Elimination of the wind factor also provides a safe workingenvironment for operating and maintenance personnel. Contamination ofthe surrounding environment (air, ground water, soil) with the highlytoxic leaching solution is also eliminated to thus materially enhancethe quality of such environment. The cost savings realized with the useof this invention have proved substantial (e.g., twenty-five cents perounce of recovered silver). It should be understood that system 10 couldbe covered with an insulative blanket, such as plastic sheets to furtherenhance the system's efficiency in certain applications.

I claim:
 1. A controlled percolation system for heap leach mining overan ore bed composed of crushed metal-laden ore deposited on aninpervious pad comprisingsupply means for conducting a leaching solutionadjacent to said ore bed, a plurality of laterally spaced tube meansconnected to said supply means for receiving said leaching solutiontherefrom; and a plurality of pre-calibrated emitter means attached toand positioned in spaced and communicating relationship along each ofsaid tube means for directly and collectively emitting said leachingsolution at predetermined flow rates and in overlapping patterns intosaid ore be at controlled and substantially uniform seepage rateswithout projecting said leaching solution to any substantial extendabove said ore bed and without causing said leaching solution to pondthereon.
 2. The system of claim 1 wherein said plurality of emittermeans emit said leaching solution at a composite average flow rateselected from the approximate range of from 0.05 to 1.50 gph/ft² of thesurface area of said ore bed.
 3. The system of claim 2 wherein saidrange is from 0.06 to 1.20 gph/ft².
 4. The system of claim 1 or 2wherein said tube means comprise a plurality of generally parallel andflexible plastic tubes laterally spaced apart one from another at adistance selected from the approximate range of from 1.0 ft. to 12.0 ft.5. The system of claim 1 or 2 wherein said emitter means are spacedapart one from another on each of said tube means at a linear distanceselected from the approximate range of from 1.0 ft. to 12.0 ft.
 6. Thesystem of claim 2 wherein each of said emitter means has a flow rateselected from the approximate range of from 0.2 gph to 6.0 gph andwherein each of said emitter means comprises an in-line emitter having ahousing, a flow control member mounted in said housing, inlet meansdefined at an upstream end of said emitter communicating with said tubemeans, outlet means defined at an upstream end of said emitter foremitting said leaching solution exteriorly of said emitter andlabyrinth-type flow passage means defined between said housing and saidflow control member and communicating said inlet means with said outletmeans for emitting said leaching solution from said outlet means bydripping at said controlled and substantially uniform seepage rate. 7.The system of claim 6 wherein said range is from 0.5 to 5.0 gph.
 8. Thesystem of claim 1 wherein said tube means are positioned on an uppersurface of said ore bed.
 9. The system of claim 1 wherein said tubemeans are at least substantially positioned beneath an upper surface ofsaid ore bed at an approximate depth of from 4.0 in. to 72.0 in.
 10. Thesystem of claim 1 wherein said leaching solution comprises an aqueoussolution of sodium cyanide or potassium cyanide or a sulfuric acidsolution.
 11. The system of claim 1 wherein the emitter means on one ofsaid tube means are staggered longitudinally relative to the emittermeans on an adjacent one of said tube means.
 12. The system of claim 1further comprising at least one second ore bed underlying saidfirst-mentioned ore bed, additional ones of each of said plurality oftube means and emitters positioned between said ore beds and means forinjecting pressurized air into at least one of said ore beds.